Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/216



Of this interesting spot it is hoped that the general section, and the views in Plate 11. fig. 2. will convey an accurate conception. Desiring to keep that description of facts which must serve as the ground-work of theory, and which seems, in the present state of science, the most useful employment of the geologist, distinct from conclusions merely speculative, I have hitherto studiously refrained from expressing the views which I have been led to form on the origin of basalt, and of the other rocks usually associated under the general name of flœtz trap.

But while describing the striking appearances presented by Kenbaan cliff, I cannot forbear to declare the conviction which this spot first impressed upon my mind, and to express my full assent to the arguments of those who maintain the igneous origin of such formations.

I would observe then that this formation is distinguished by characters so directly opposed to those which all rocks undoubtedly of aqueous origin possess, that no hypothesis which ascribes both to a common origin, can be otherwise than contradictory, and at variance with itself. For

I. Of all other formations, the least ancient are the least elevated; but this, the most recent of all, yet rivals the primitive mountains in height.

2. Of all other formations, the degree of consolidation decreases together with its age, their texture passing from crystalline through the several gradations of sub-crystalline, compact, coarse, and lastly earthy; while in this formation, even where it rests on chalk, the crystalline texture of the oldest rocks frequently recurs.

3. Whin dykes, which are indisputably connected with this formation, differ from all other mineral veins, in the circumstance of their traversing all rocks indifferently; while of other veins, particular classes are exclusively associated with particular rocks.

Such being the negative evidence against the Neptunian hypothesis, I proceed to that which is positive in favor of the volcanists; as

1. The identity of chemical composition in basalt and lava.

2. The constant occurrence of tr-ap rocks in volcanic districts.

3. The confession of the Wernerians themselves, that the basalt of Auvergne is of igneous origin.

4. The testimony of those best acquainted with districts still exhibiting active volcanoes. Such persons, as Dolomieu and Spallanzani, have uniformly maintained the igneous origin of basalt, while those who have contended against it have generally been unacquainted with countries of this description.

Having thus alluded to, rather than stated, some of the general arguments on which this question appears to me to depend, I return to Kenbaan, where the basalt is seen extending from beneath as well as overlying the great mass of chalk, which has at one extremity assumed such a curvature as would naturally result from lateral pressure; and at the other is rent, and shattered in the most extraordinary manner, the basaltic matter insinuating itself into the fissures, and often converting by its contact chalk into granular marble, while fragments of the chalk, of all sizes, appear to have been forced upwards and imbedded in the basaltic rock, having suffered in their superficial parts where the basalt touches them a most remarkable change.

It seems impossible to conceive appearances more utterly irreconcilable with the hypothesis, that the basalt was deposited regularly above the chalk from a state of aqueous solution. On the other hand, were we to imagine a. priori, the phenomena which would probably result from the eruption of a current of ignited lava from beneath the chalk, and its subsequent diffusion over the upper surface of the chalk, while the whole was submerged beneath the sea, and under a considerable pressure, they would exactly accord with those which may actually be observed at Kenbaam.

To the same purpose the changes effected by the whin dykes of this district on the rocks they traverse might be cited. Thus we have instances:

1. Of the conversion of old red sandstone to hornstone. See page 201.

2. Of the conversion of the slate clay of the coal measures to flinty slate, and of the reduction of the coal itself to cinders. See pages 205, 206.

3. Probably, also, of the conversion of the slate clay of the lias formation into flinty slate. See page 213.

4. Of the conversion of chalk in several places into granular marble. See pages 172. 173.

Hence, if it be allowable to speculate on subjects so remote from actual observation, I would infer that the hypothesis which ascribes the formation of the theta trap rocks to submarine volcanoes, which were active at a very remote period before the seas and continents had assumed their present relative level, is both in itself more consistent, and in its application to the actual phenomena more satisfactory than any other.

It is evident that the basaltic mass of Ulster was accumulated antecedently to the last great convulsion which has modified the surface of our globe, excavating its rallies, and constituting its alluvial deposits.